Callsigns on SDR Waterfall

We recently received a question from someone watching a websdr waterfall reporting that they could “see” callsigns in the waterfall, wondering how this was done. As it’s quite cool, we thought we’d explain in basic terms what’s going on.

Why is this done?

Mainly because it looks cool. Many of us now use SDRs and have a waterfall view of the band – seeing scrolling text or a visual callsign is eyecatching, and something different.

How is this done?

The concept is broadly similar to SSTV – you take an image of some letter, convert it into a digital format, and modulate the RF signal so that the image appears in the waterfall. Several digital mode software packages offer this option, with the two most common probably being EasyPal and FLDigi

If you want to know more, there’s an interesting walkthrough video created by Pavel OM0ET, exploring how this works, and showing a cool way to send your callsign visually.


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